A primary objective in the care of patients having respiratory problems is to reduce the work of breathing and to provide supplemental oxygen to the patient. There are a number of devices, referred to herein as oxygen dilution devices, available for the clinician to provide to the patient oxygen in the range of from about 24% to 100%. The amount of oxygen required depends upon the diagnosis of the patient and blood gas analysis. These devices normally utilize the Bernoulli principal and are designed to provide a precise inspired oxygen concentration from 24% to 60% at high total gas flow. It is important that total gas flow be maintained at a high level in order to meet the respiratory demands of the patient so that the delivered concentration of oxygen is not diluted secondarily by abnormally high minute ventilation.
Most of the oxygen dilution devices presently available provide means for adjusting the concentration of oxygen in the air stream exiting the device. These devices generally comprise a body in which the primary oxygen stream is received and which is provided with one or more ports which open to the exterior of the body for the intake of ambient air. The flow of the primary oxygen stream through the dilution body decreases the pressure within the body causing ambient air to be drawn into the body to mix with the oxygen stream and passed on to the patient. By varying the size of the air intake port to increase or reduce the flow of air into the diluter, the concentration of oxygen in the air stream can be varied. Conventionally, the dilution devices include a plurality of ports to the exterior which vary in size and which are provided with a covering device so that a port of desired size is opened to the exterior while the other ports are closed. Examples of various designs of oxygen dilution devices are shown in U.S. Pat. No. 3,794,072, issued Feb. 26, 1974 to Diedrich et al; U.S. Pat. No. 3,850,171, issued Nov. 26, 1974 to Ball et al; U.S. Pat. No. 3,913,607, issued Oct. 21, 1975 to John Price; U.S. Pat. No. 3,977,432, issued Aug. 31, 1976 to Vidal; and U.S. Pat. No. 4,848,333, issued Jul. 18, 1989, to Richard B. Waite. As illustrated in these patents, the oxygen dilution device is incorporated in the air hose leading to a mask. The primary oxygen stream is introduced into the hose and air is drawn in due to the reduced pressure of the oxygen flowing through the dilution device to dilute the primary oxygen stream prior to reaching the patient.
A primary problem with these devices occurs when there is an obstruction in the passage leading which produces a restriction of the total gas and a back pressure in the diluter body. In this case, the flow of the primary stream, which is under pressure, remains essentially constant while the pressure in the diluter increases due to the back pressure thus reducing air intake. This results in an enrichment of oxygen in the diluted stream reaching the patient which may lead to injury to an oxygen sensitive patient, such as one suffering with emphysema, and at the least results in a loss of control over the concentration of oxygen delivered to the patient. Such downstream resistance may be caused by a small amount of phlegm or condensation in the line which is sufficient to produce a back pressure and resultant increase in oxygen concentration to the patient. In addition, most dilution devices used in respiratory care are attached to a face mask in which large openings are provided for the egress of the high flow air stream which is not inhaled and to prevent the build-up of CO.sub.2. Although practitioners often assume that the concentration settings of the dilution device accurately indicate the true concentration of oxygen delivered to the patient, this is not a valid assumption because of back pressure created at the face mask by the high total gas flow. As a result, the respiratory clinician and/or attending nurses must be constantly on alert to such a situation and the inhalation system as well as the blood gases must be monitored if dilution devices are to be used with any degree of confidence.